Spheno-occipital synchondrosis closes at the age of:

The spheno-occipital synchondrosis contributes most to growth of the posterior cranial base as it is the last cranial base suture to ossify. The spheno-occipital synchondrosis closes by 13–15 years of age. Growth at the spheno-occipital synchondrosis causes elongation of the middle portion of the cranial base as a result of primary displacement, while growth at the posterior part leads to flexion in the cranial base, since the posterior part of spheno-occipital synchondrosis has a greater amount of bone formation in its inferior part than its superior part. Key Concept: Anterior and posterior cranial base lengths increase because of bony deposition, as well as growth at the spheno-occipital and sphenoethmoidal synchondroses. Postnatally, the posterior cranial base becomes longer primarily due to growth at the spheno-occipital synchondrosis. Ref: Orthodontics: Diagnosis and Management of Malocclusion and Dentofacial Deformities 3rd ed pdf no 654